The need for cholesterol is high in maturing brain, when various CNS structures such as synaptosomes and myelin are being formed, and is low after maturation. Since cholesterol in brain is derived entirely from endogenous synthesis, the cholesterol synthesis in brain must be regulated strictly according to its need. This therefore presupposes the existence of a metabolic regulator(s) or pacemaker(s) for cholesterol synthesis within the central nervous system, which is responsive to certain endogenous or exogenous changes accompanying maturation. The main objective of our research project is to identify the rate-limiting reactions for cholesterol synthesis in brain and to determine how these reactions are affected by exogenous or endogenous changes accompanying brain maturation. To accomplish this we propose to measure the relative rates of individual reactions within the multi-enzyme system for conversion of acetate to mevalonate, mevalonate to squalene and squalene to cholesterol, at various stages of development. The role of cofactors and vailability of substrates in the rate-limiting process will also be investigated. Additionally, we will determine whether in hypomyelination there is a further attenuation of normally rate-limiting processes or whether there is a different mechanism of inhibition. Esterified cholesterol is a minute constituent of normal adult brain. A transient increase in cholesterol esters occurs just before the onset of active myelination. It is also well established that the concentration of esterified cholesterol is high in a variety of demyelinating conditions. These facts suggest a role for cholesterol esters in myelination and in demyelination processes. Our other objective is to gain an understanding of the mechanism of increase in cholesterol ester formation just prior to myelination and in experimental demyelination. This will include the determination of the activities of cholesterol esterifying enzymes as well as cholesterol ester hydrolases. Additionally, we will compare the exchange rate of free and esterified cholesterol within the brain subcellular fractions.